1. Field of the Invention
The invention relates to a vibration damper with a pressure stop where the vibration damper includes a cylinder filled with a damping medium in which a piston rod together with a first piston is installed with freedom of axial movement and where the piston divides the cylinder into first and second working spaces.
2. Description of the Related Art
A piston-cylinder assembly with a hydraulic-mechanical pressure stop is disclosed in known from DE 198 29 765 A1, which is hereby incorporated by reference in its entirety. Therein, a second piston is pushed by a rigid extension of the piston rod against a return spring as soon as the piston rod has traveled inward by a certain distance. The second piston is guided inside a pressure cylinder, which has a number of throttle connections leading to the adjacent working space and which is permanently supported on a valve body at the base of the vibration damper.
During a normal inward travel phase, only the throttle connections between the second piston and the base of the pressure cylinder produce the damping force. A disk valve inside the separating piston opens only when peaks in the damping force exceed a certain threshold. This type of pressure stop can be used effectively only in heavy trucks or other types of construction vehicles, in which an extreme pressure stop effect is to be achieved. As a matter of principle, the damping force characteristic of a pressure stop with a fixed throttle cross section is parabolic. This damping force characteristic, however, cannot be usefully adapted to a conventional passenger vehicle. That is, either both the damping force which occurs at low piston rod speeds and the maximum damping force in the desired range are too low, or the damping force is suitable at lower elastic travel speeds but much too high at faster piston speeds.
In applications in conventional passenger vehicles, furthermore, it is also disadvantageous that the point at which the hydraulic pressure stop goes into action can be easily detected. Entrance grooves maybe made in the second piston to provide a bypass over a certain part of the distance traveled by the separating piston, but the effectiveness of this measure is limited.
In present invention a hydraulic pressure stop provides a gentle transition between the range of normal travel distances and the range in which the pressure stop goes into effect. The pressure stop offers at least the possibility of a degressive damping force characteristic.
In the present invention, the movement of the piston rod acts by way of a transfer spring on the second piston, and in that all of the displaced damping medium is displaced from the third working space through the disk valve of the second piston into the second working space.
Because of the incorporation of the second piston into the design of a pure disk valve, the hydraulic pressure stop can be adapted very precisely to the requirements, especially to the requirements for a passenger vehicle, which requires comfortable damping behavior. The damping force of the first piston is sufficient over the range of displacement, i.e. stroke, distances traveled at small useful loads. The damping force of the second piston is added to that of the first piston only after a defined load state has been exceeded. It, therefore, becomes possible to adjust the damping force characteristic of the first piston more carefully to the requirements of comfort. The reason for this is that, because of the use of the disk valve, the second piston already provides a significant amount of initial damping even at relatively low flow rates without become too xe2x80x9chardxe2x80x9d at high flow rates.
So that the damping force can be initiated smoothly, it is advisable for the elastic force of the transfer spring to be greater than the elastic force of the return spring, at least after the piston rod has traveled a defined distance after starting to transmit force to the second piston. Otherwise, in cases where the transfer spring is very soft, it is possible for the transfer spring to be compressed into a solid block before the second piston starts to move.
Advantageously, the vibration damper may have a separating piston, which separates a compensating space from the third working space. Therein, the return spring is supported on the separating piston. The second piston is supported in practice against the pressure in the compensating space, which can be generated by a gas cushion or by a pretensioning spring. This pressure is transmitted to the third working space.
So that the return spring can be held in a defined position even when the first piston is traveling outward, the separating piston is provided with a tensile force locking device for the return spring, so that the end of the return spring facing the separating piston always moves synchronously with the separating piston.
Thus, the tensile force locking device is advantageously formed by a mushroom-shaped part on the surface of the separating piston, which is gripped by a turn of the return spring.
The transfer spring remains connected in turn to the second piston regardless of the distance it travels. The separating piston, the return spring, and the transfer spring can be preassembled to form a unit independently of the other components of the vibration damper.
Many different materials can be used to fabricate the transfer spring. It can be effective, for example, for the transfer spring may be formed by an elastomeric body.
Especially when the transfer spring is very soft, it is effective for the second piston to have a guide sleeve for the transfer spring to ensure good radial guidance.
The second piston may also have a check valve, which opens in the direction of the third working space. This prevents the third working space from being unable to accept damping medium quickly enough at high rates of elastic travel.
As a further design feature, the disk valve and the check valve are clamped by a fastening means to the second piston, where the fastening means has an axial locking device for the return spring.
The first piston requires only a few adjustment measures, because the transfer spring is supported on a support plate for at least one valve disk of the first piston.
So that the support plate does not exert a negative effect on the desired damping force characteristic of the two pistons, the support plate is provided with a number of flow-through openings for a non-throttling connection between the second working space and the valves in the first piston.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.